High-temperature Methane Research Articles

Diagnosing the Nature of Oxygen Site in Yttrium Doped Barium Calcium Niobates and Its Impact Towards Electrochemical Oxidative Coupling of Methane

Effective conversion of methane into value added chemicals such as ethylene will help reduce its venting and flaring from oil and gas wells. Oxidative coupling of methane (OCM) is shown to produce ethylene although over oxidation products such as CO2 and CO reduce the C2 hydrocarbon selectivity. The industrial target for OCM process is a single pass conversion of at least 30% and a C2 selectivity around 80% although catalysts with satisfactory performance has not been identified yet despite four decades of intense research.1 Electrochemical OCM is recently gaining renewed interest due the ability to control the conversion rate and product selectivity though temperature and applied potential. We demonstrated a potential dependent C2 selectivity using Fe and Ca codoped barium niobates as E-OCM catalysts.2–4 However, introduction of yttrium as a dopant in barium calcium niobate (BCNY) resulted in a remarkable change in its electrical conductivity and methane activation properties. We present our results on BCNY’s methane activation properties and possible correlation between the nature of oxygen species in the BCNY crystal matrix. Electrical conductivity measurements, E-OCM measurements and temperature programed reactions in oxidizing and reducing conditions are utilized to establish the role of yttrium and compare with other transition metal dopants such as Fe and Ni.References Zavyalova U, Holena M, Schlögl R, Baerns M. Statistical Analysis of Past Catalytic Data on Oxidative Methane Coupling for New Insights into the Composition of High-Performance Catalysts. ChemCatChem. 2011;3(12):1935-1947. doi:10.1002/cctc.201100186Ramaiyan K, Denoyer LH, Benavidez A, Garzon FH. Role of Dopants in Fine-Tuning the Electrical and Catalytic Properties of Barium Niobate Perovskites Towards High-Temperature Electrolyzer Application. ECS Trans. 2023;111(6):1139. doi:10.1149/11106.1139ecstDenoyer LH, Benavidez A, Garzon FH, Ramaiyan KP. Highly Stable Doped Barium Niobate Based Electrocatalysts for Effective Electrochemical Coupling of Methane to Ethylene. Adv Mater Interfaces. 2022;9(27):2200796. doi:10.1002/admi.202200796Denoyer LH, Benavidez A, Brearley A, Ramaiyan K, Garzon FH. Chemical Stability of BaMg0.33Nb0.67-XFexO3-δ in High Temperature Methane Conversion Environments. ECS Trans. 2023;111(6):587. doi:10.1149/11106.0587ecst

A and B Site Doped Barium Niobates with Improved Electrical Properties of Electrochemical Oxidative Coupling of Methane

Electrochemical oxidative coupling of methane (E-OCM) to produce ethylene is gaining renewed interest due the ability to control the conversion rate and product selectivity though temperature and applied potential. We previously demonstrate Mg, Fe and Ca, Fe codoped barium niobates have remarkable methane activation properties coupled with good chemical stability. Nevertheless, their electrical conductivity remained less than 50 mScm-1 that is low for electrode application. Hence, we attempted both A and B site doping on barium niobates with various dopants such as Sr, La (A site), and Ca, Mg, Fe, Co, Ni, Y (B site) to improve their electrical properties. These doped niobates showed enhanced thermochemical stability in SOFC relevant conditions and catalytic activity towards methane activation. The redox behavior of the Nb4+/5+ couple seem to be at a key reason behind this redox stability while the size and electronegativity of the dopants affect the electrical properties. The chemical stability was analyzed by TGA measurements followed by analysis of the perovskite powders using PXRD measurements. Impedance measurements under difference gas environments were utilized to analyze their electrical conductivity. Chronoamperometric measurements were utilized to evaluate the E-OCM properties of these catalysts. Our results demonstrate doped barium niobates as a promising candidate for stable operation in high temperature electrochemical applications. References H. Denoyer, A. Benavidez, A. Brearley, K. P. Ramaiyan*, F. H. Garzon*, “Electrochemical oxidative coupling of methane: deciphering the exceptional properties of BaMg0.33Nb0.67-xFexO3-δ for enhanced electrocatalysis and durable operation” Under revision J. Electrochem. Soc, (December 2023).H. Denoyer, A. Benavidez, F. H. Garzon*, K. P. Ramaiyan*, “Highly Stable Doped Barium Niobate Based Electrocatalysts for Effective Electrochemical Coupling of Methane to Ethylene” Adv. Mater. Interfaces, 2022, 2200796 (1-9). Ramaiyan, L. H. Denoyer, A. Benavidez, F. H. Garzon*, “Role of Dopants in Fine-Tuning the Electrical and Catalytic Properties of Barium Niobate Perovskites Towards High-Temperature Electrolyzer Application” ECS Transactions 2023, 111, 1139.H. Denoyer, A. Benavidez, A. Brearley, K. Ramaiyan, F. H. Garzon*, “Chemical Stability of BaMg0.33Nb0.67-XFexO3-δ in High Temperature Methane Conversion Environments” ECS Transactions, 2023, 111, 587.

Dynamic trap of Ni at elevated temperature for yielding high-efficiency methane dry reforming catalyst

Highly dispersed and stable metal catalysts with small nanoparticles have received extensive attention in elevated-temperature thermocatalytic process. However, the available strategies to stabilize metal sites, by constructing defective structures on catalyst supports and developing controllable preparation steps at room temperature, show limited effect, because these active metal sites can be mobile and sintering at elevated temperature. Herein, dealuminated Beta zeolite with abundant surface defects of silanol nests is applied as support, then dynamic trap strategy and subsequent reduction process at elevated temperature is devoted to transfer Ni-based precursors into the silanol nests, thus obtaining small nanoparticles Ni catalysts that are suitable for high-temperature methane dry reforming (DRM). Some in-situ characterization processes and the ingenious designed experiments are performed to identify the dynamic trapping process. The rational fabricated catalysts exhibit high catalytic reactivity for DRM reaction in long-term operation.

Role of Dopants in Fine-Tuning the Electrical and Catalytic Properties of Barium Niobate Perovskites Towards High-Temperature Electrolyzer Application

Perovskite materials are being studied for high temperature electrochemical applications such as solid oxide fuel cells (SOFC) and electrolyzers due to their tunable conductivity and catalytic activity. However, high temperature operation poses significant challenges in both fabrication and durable operation that is further complicated by the operating environment.1 We studied barium niobates with various dopants such as Fe, Ni, Ca, Y, and Mg for B site doping while Sr, K, and La are studied for A site doping. These niobates showed enhanced chemical stability in SOFC relevant conditions and catalytic activity towards methane activation. Nb4+/5+ redox couple seem to be a key reason behind this chemical stability while the size and electronegativity of the dopants affect the electrical properties.2 Catalyst exsolution is explored with dopants such as Fe, Ni, and Co under reducing conditions along with surface bound metallic nanoparticles prepared by an incipient wet synthesis method. Chemical stability of these perovskites was analyzed by an in situ TGA measurements under various gas environments followed by analysis of the powders using PXRD, XPS, SEM, and TEM methodologies.3 Methane activation properties are further analyzed in a temperature programmed reaction setup while Electrochemical properties were evaluated in a home-made setup using different electrolyte materials in the temperature range of 700°C to 900°C. Cyclic voltammetry, impedance spectroscopy, and chronoamperometry measurements coupled with continuous mass spectra analysis was utilized to diagnose any correlation between product distribution and applied potential. Our results demonstrate doped barium niobates as a promising candidate for stable operation in high temperature electrochemical applications. The presentation will discuss these results and provide a correlation between dopants and catalytic activity and chemical stability of doped barium niobate perovskites. References (1) Ramaiyan, K. P.; Denoyer, L. H.; Benavidez, A.; Garzon, F. H. Selective Electrochemical Oxidative Coupling of Methane Mediated by Sr2Fe1.5Mo0.5O6-δ and Its Chemical Stability. Commun. Chem. 2021, 4 (1), 139. https://doi.org/10.1038/s42004-021-00568-1.(2) Denoyer, L. H.; Benavidez, A.; Garzon, F. H.; Ramaiyan, K. P. Highly Stable Doped Barium Niobate Based Electrocatalysts for Effective Electrochemical Coupling of Methane to Ethylene. Adv. Mater. Interfaces 2022, 9 (27), 2200796. https://doi.org/10.1002/admi.202200796.(3) Denoyer, L. H.; Benavidez, A.; Garzon, F. H.; Ramaiyan, K. P. Chemical Stability of BaMg0.33Nb0.67-XFexO3-δ in High Temperature Methane Conversion Environments. under communication - ECS Journal of Solid State Science and Technology.

CFD modeling of subsonic and sonic methane gas release and dispersion

AbstractIn the event of a flammable liquid, gas, or vapor release the first step is to identify the type of outflow, which can fall into two categories sonic or subsonic. The two types of outflows carry different flow characteristics, which effect on the extent of the potentially explosive areas. In case of subsonic outflow, a short jet is formed without turbulent flow conditions at low velocity, which appears more concentrated around the source of release. With sonic outflow, a high velocity jet is formed with turbulent flow properties, which can extend further away from the source of release. The simulations examine the lower explosion limit of the flammable medium around the vessel where LEL20% or LEL40%. In addition, high temperature methane gas release was also presented.

High temperature methane emissions from Large Igneous Provinces as contributors to late Permian mass extinctions

Methane (CH4) emissions induced by Large Igneous Provinces have the potential to contribute to global environmental changes that triggered mass extinctions in Earth’s history. Here, we explore the source of methane in gas samples from central Sichuan Basin, which is within the Emeishan Large Igneous Province (ELIP). We report evidence of high methane formation temperatures (between 249−17/+19 and 256−20/+22 °C) from clumped methane measurements and mantle-derived signatures of noble gases, which verify that oil-cracked CH4 and pyrobitumen are by-products within the reservoirs, associated with hydrothermal activity and enhanced heating by the ELIP. We estimate the volume of oil-cracked CH4 induced by the ELIP and argue that CH4 emissions would have been sufficient to initiate global warming prior to the end of the Permian. We also suggest that similar emissions from oil-cracked CH4 associated with the Siberian Traps Large Igneous Province may also have contributed to the end-Permian mass extinction significantly.

Experimental investigation of the high temperature chlorination of methane in a tubular reactor

High temperature methane chlorination offers an alternative process route to the utilization of methane, since the presence of chlorine lowers the pyrolysis temperature. Chlorine, methane and nitrogen were introduced in a tubular reactor while varying hydrogen to chlorine ratios at temperatures ranging from 600 °C to 1000 °C. The chlorine conversion rates, hydrogen chloride yields, and the chlorine content in the soot were determined. While complete conversion of the limiting reactant was found in most experiments, differing trends were observed for the hydrogen chloride yield. Chlorine was almost always found in the carbon. At the same methane flow rate, a tendency for carbon formation was ascertained to depend on the amount of chlorine supplied, with the most extensive deposits being formed for a hydrogen to chlorine ratio of unity. Furthermore, in the case of high chlorine excess, crystalline products form at the reactor outlet.

Electrospun nanofibrous Ni/LaAlO3 catalysts for syngas production by high temperature methane partial oxidation

Ni/Al2O3 catalysts have been widely used for methane reforming while the formation of NiAl2O4 with low reducibility reduces catalyst efficiency. La2O3 was used to promote the catalytic activity of Ni/Al2O3 catalysts through improving Ni dispersion. LaAlO3 perovskite showed catalytic activity in methane coupling and also used as a catalyst support for methane reforming. This study systematically investigated the effect of La2O3 addition into Ni/Al2O3 catalysts and found the formation of LaAlO3 perovskite played an important role, which requires high crystallization temperatures. The thermally-stable structure of nanofibrous catalysts was employed to develop high-performance Ni/LaAlO3 catalysts. High calcination temperature resulted in the enhanced crystallinity of LaAlO3 perovskite, improved Ni reducibility and strengthened catalyst/support interaction, which contributed to high catalytic performance during methane partial oxidation. The Ni/LaAlO3 catalyst calcined at 1100 °C generated a CH4 conversion of 91.2% during methane partial oxidation with H2 and CO selectivities of 95.5% and 92.4%, respectively. It is because La2O3 addition into Ni/Al2O3 promoted Ni reduction via forming LaAlO3. Therefore, an efficient and thermally-stable fibrous Ni/LaAlO3 catalyst has been developed for high temperature methane partial oxidation.

Preparation of methanation catalysts for high temperature SOEC by β-cyclodextrin-assisted impregnation of nano-CeO2 with transition metal oxides

The aim of this work was to prepare and examine the catalytic activity of nanometric CeO2 decorated with transition metal oxides – Ni, Co, Cu, Fe and Mn – towards a high-temperature methanation process under SOEC CO2/H2O simulated co-electrolysis conditions. Samples were prepared using the wet impregnation method via the conventional process and with the addition of native cyclodextrin. The influence of β-cyclodextrin (βCD) onto the size, dispersion and integration of the obtained metal nanoparticles was investigated. The differences between the catalysts’ reducibility revealed that samples prepared from βCD-containing solutions, in most cases, resulted in the creation of smaller MexOy NPs on the surface of the substrate material compared to those prepared using traditional nitrate solutions. The samples containing Ni and Co were the only ones that observably catalysed methane synthesis. The high dispersion and integration of NPs prepared via the proposed synthesis route resulted in increased catalytic activity and enhanced stability, which was most pronounced for the Co-impregnated sample. The methane production peak for Ni-βCD/CeO2 at 375 °C was characterised by nearly 99% CO conversion and 80% selectivity towards CH4 production. Co-βCD/CeO2 reached 84% CO conversion and almost 60% methane selectivity at 450 °C. The usage of CeO2 coupled with βCD for the preparation of catalysts for high-temperature methane synthesis for use in SOECs gave promising results for further application.

Heteroepitaxial growth of diamond films on Y3Al5O12 single crystals

Up to now, it is still challenging to solve the thermal management problem of the solid-state laser. Herein, a new thermal management method is conducted, which is direct deposited diamond films on laser crystals. By microwave plasma chemical vapor deposition (MPCVD) method, we have successfully synthesized high-quality diamond films on Y3Al5O12 (YAG) substrates to improve the thermal performance of YAG crystals. Diamond films were grown with different pre-treatments and deposition parameters, where the pre-treatment method is to expose the substrate to diamond growth conditions before the seeding step. After deposition, the quality, thermal properties, and optical properties of as-grown samples were investigated in detail to study the impact of the pre-treatment and deposition parameters on the properties of the film. The results revealed that a sufficient pre-treatment process is necessary for the deposition of continuous diamond films on YAG substrates, while a relatively high temperature and low methane concentration during the deposition process are conducive to obtaining high-quality diamond films. Furthermore, it was found that depositing diamond films on YAG crystals can effectively improve the thermal conductivity of the materials with slightly affecting their optical properties, which provides a new choice for the thermal management of the solid-state laser.

Dual frequency comb absorption spectroscopy of CH4 up to 1000 Kelvin from 6770 to 7570 cm-1

As infrared telescopes are finding evidence of methane in hot exoplanet atmospheres, it is becoming increasingly important to have accurate high-temperature methane absorption models. To evaluate and update several spectroscopic databases (HITRAN2016, HITEMP, ExoMol), we collected laboratory spectra of the methane icosad (6770–7570 cm-1) using a 200 MHz (0.0067 cm-1) dual frequency comb spectrometer. The pure methane data span 18 to 300 Torr and 296 to 1000 K. We found good agreement with the HITRAN2016 model spectrum at room temperature, and substantial mismatch between our spectra and all absorption models at elevated temperature. We present several updates to HITRAN2016 to improve agreement with the measured high-temperature spectra. Specifically, we assign 4283 lower-state energies which had previously been given a default value in HITRAN2016 (EHIT″= 999 cm-1), update existing lower-state energies for 92 features, and add 293 new high-temperature features in order to improve HITRAN2016 above 300 K. Additionally, we update the band-wide line positions by ∼0.001 cm-1, the self-widths by +7%, and we estimate band-averaged temperature-dependence exponents for self-width (0.85) and self-shift (0.58). These measurements are an important step towards merging empirical and theoretical high-temperature methane databases, with the goal of enabling better understanding of exoplanet atmospheres.

Ambient pressure CO2 hydrogenation over a cobalt/manganese-oxide nanostructured interface: A combined in situ and ex situ study

We report on a cobalt/manganese-oxide interface catalyst with outstanding activity and selectivity towards methane even at high temperatures and ambient pressure in CO2 hydrogenation. The catalyst was formed from a MnCo2O4-based spinel structure during the oxidative-reductive pretreatment process just before the catalytic tests. Several Mn-, Fe- and Ni-containing cobaltite spinel and reverse spinel structures were tested to find the best overall performer. The reusable MnCo2O4-based structure featured a CO2 consumption rate of ~8500 nmol*g−1*s−1. Even though methane is not the thermodynamically favoured product, it was produced with ~80% and ~50% selectivity at ambient pressure at 673 K and 823 K, respectively. This unexpected finding is linked to the presence of a unique nanostructured Co/Mn(II)O catalyst with a surface composition of Mn3.3Co2.0O4.7 formed after the pretreatment activation step. Over this phase, the reduction of CO2 progresses through bridge bonded formate located at the Co/Mn2+ interface and this is mostly responsible for high temperature methane formation. This hypothesis is proven here by the reported combination of ex-situ XRD, TPR, HRTEM-ED, HAADF-EDX and in-situ NAP-XPS and DRIFTS techniques.

Tin oxide gas sensor on tin oxide microheater for high-temperature methane sensing

Despite the ever-increasing demand for methane detectors in residential and industrial locations, a long life semiconductor-based methane sensor is yet elusive. The problem is rooted in the high decomposition temperature of the methane molecule. Here, we disclose a tin oxide gas sensor heated up to its operating temperature using a ten micron-thick undoped tin oxide microheater which can operate at temperatures as high as 850 °C, sufficient for the spontaneous pyrolysis of methane. Operating at temperatures above 700 °C, the device selectively senses methane in atmospheres contaminated with CO and H2. The response and recovery times are both ~10 s, and the CH4 detection limit is 50 ppm, rendering the developed sensor appropriate for online measurement of methane level in hot exhaust gases. Operating continuously at 700 °C in air, the device exhibits no noticeable aging in a test duration of 30 days.

Solid carbon production and recovery from high temperature methane pyrolysis in bubble columns containing molten metals and molten salts

We study methane pyrolysis in a bubble column reactor consisting of a layer of molten NiBi alloy, and a molten salt layer floating on the metal layer. The molten metal is a pyrolysis catalyst; the molten salt layer is added for removing contaminants from the carbon product. Methane is introduced at the bottom of the reactor and the rising bubbles contain reactant and product gases together with metal vapor and flakes of solid carbon. When the bubbles rise through the molten salt layer the metal vapor condenses producing dense liquid metal droplets, which sink to the metal phase; the low-density carbon rises to the top of the salt layer. The carbon generated from the single-phase molten NiBi reactor has a different structure from that produced in the two-phase NiBi/salt reactor. The amount of metal in the carbon product is less than 5 wt %, in the two-phase reactor, compared to 83 wt % in the NiBi single-phase reactor. After subsequent purification steps one obtains solid carbon with less than 2 wt % salt contamination and no detectable metal contamination.

Nickel catalysts supported on La2O3-modified KIT-6 for the methane dry reforming reaction

Nickel catalysts are vulnerable to suffering from carbon deposition and metal sintering in the methane dry reforming process under high temperature. Based on these problems, in this paper, La2O3–KIT-6 materials with different La2O3 contents were designed and prepared in order to take advantage of the basic sites of La2O3 and rich mesoporous structure of KIT-6, and then nickel catalysts supported on La2O3–KIT-6 materials were synthesized. In contrast, the Ni/KIT-6 catalyst was also prepared by the same method. XRD, TEM, SEM, physisorption of N2, XPS, H2-TPR, Raman, and TG were used for fresh and spent catalysts characterization in this work. The catalytic performance of these Ni/La2O3–KIT-6 catalysts was tested under high temperature methane dry reforming reaction. Our work showed that the La2O3-modified catalysts showed reasonably superior catalytic activity and high long term stability compared with the nickel catalyst supported on KIT-6 without modification of La2O3. The high dispersion of nickel nanoparticles on La2O3–KIT-6 as well as the basic property of La2O3 were both helpful to the resistance towards carbon deposition, and the interaction between Ni nanoparticles and support material in Ni/La2O3–KIT-6 was helpful to the resistance towards nickel sintering on account of the special physicochemical properties of La2O3.

High-temperature laminar flame speed measurements in a shock tube

High-temperature methane and propane laminar flame speed measurements were conducted behind reflected shock waves in a shock tube. A high-power Nd:YAG laser was used to spark-ignite the shock-heated gas mixtures and initiate laminar flame propagation. High-speed, OH* endwall imaging was used to record the propagation of the spherically expanding flames in time, and a non-linear stretch correlation was applied and used to determine the unburned, unstretched laminar flame speed. “Low-temperature” (<600 K) flame speed results are presented for stoichiometric methane/air and propane/air mixtures at initial unburned gas conditions of 489–573 K and 391–556 K, respectively, and 1 atm. The low-temperature measurements show close agreement with available literature data and kinetic modeling results, thereby validating the shock-tube laminar flame speed measurement approach. “High-temperature” (>750 K) flame speed results are presented for a propane/21% O2-47% N2-32% He mixture (ϕ = 0.8) at initial unburned gas conditions of 764–832 K, 1 atm. The high-temperature measurements fall between kinetic model predictions, but the kinetic model results show significant disagreement, highlighting the need for high-temperature flame speed validation data of this kind. We believe that these results represent the first laminar flame speed measurements conducted in a shock tube, and that the high-temperature results are the highest-temperature, 1-atm flame speed measurements available in the literature.

Atlas of Experimental and Theoretical High-temperature Methane Cross Sections from T = 295 to 1000 K in the Near-infrared

Abstract Spectra of hot methane were recorded using a tube furnace and a high-resolution Fourier transform spectrometer. We obtained experimental absorption spectra in the 1.85–1.11 μm near-infrared region at eight temperatures ranging from 295 K up to 1000 K. We have converted these into an atlas of absorption cross sections at each temperature for the methane tetradecad, icosad and triacontad polyads, excluding some spectral intervals either strongly contaminated by water or due to baseline fringes. On the theoretical side, the spectra were simulated from the ab initio-based Reims-Tomsk TheoReTS line list for the same experimental conditions. This line list has been constructed by global variational calculations from potential energy and dipole moment surfaces followed by empirical line position corrections deduced from previously published analyses. The comparisons showed very good overall agreement between observations and theory at high spectral resolution for the tetradecad and icosad and at medium or low resolution above this range. A full set of the theoretical absorption cross sections is also included. Detailed temperature dependence of the methane absorption enables the efficient method of remotely probing the temperature of distant astronomical objects based on a comparison of relative signals in carefully selected spectral intervals. This first combined experimental and theoretical easy-to-use cross-section library in the near-infrared should be of major interest for the interpretation of current and future astronomical observations up to a resolving power of 100,000–300,000 in the range 6400–7600 cm−1 and a resolving power of 5000–10,000 in the higher wavenumber range up to 9000 cm−1.

Unique spatial methane distribution caused by spontaneous coal combustion in coal mine goafs: An experimental study

Abstract A new type of incident in the form of a methane explosion caused by spontaneous combustion of coal is receiving more and more attention due to the high ground temperature and high methane content in coal mines. To investigate the formation process of this type of incidents and advance the research on methane explosion risk caused by spontaneous coal combustion, a self-designed experimental platform was used to determine the influence of spontaneous coal combustion on methane migration in the goaf. Additionally, the effect of different ventilation velocities at the mining face on methane migration in the goaf was studied. The results reveal that air leakage plays a key role in methane migration in the goaf. The bigger the ventilation velocity, the larger the area affected by air leakage. In addition, the superposition of the air leakage and the horizontal and vertical movement of hot gases creates the entrainment effect in the spontaneous combustion area. Eventually, the methane concentration at the location of spontaneous combustion is the lowest and it gradually increases outside the spontaneous combustion area because of the superposition of air leakage, ‘fire pressure’ and the entrainment effect.

Metastable Pd ↔ PdO Structures During High Temperature Methane Oxidation

Methane in the form of natural gas is increasingly used as a transportation fuel, but the treatment of methane in the exhaust is a challenge since methane is a potent greenhouse gas. Pd is one of the most active catalysts for methane oxidation. Previous work has shown that transformation of Pd into the oxide, and decomposition of the oxide to metallic Pd can occur as temperature is raised in an oxidizing atmosphere, causing profound changes in catalytic reactivity. Equilibrium thermodynamics predict that the phases Pd and PdO must be in equilibrium at a well-defined temperature and oxygen pressure, since the two phases are immiscible and do not form solid solutions. But catalytic data suggests the existence of metallic Pd under conditions where only PdO should be thermodynamically stable. In this study we have explored the Pd ↔ PdO transition at high temperature using in situ XRD, TGA and from TEM examination of Pd catalysts that were quenched in liquid nitrogen or in a heating TEM holder to prevent any changes in microstructure during cooling. Corresponding data was obtained during methane oxidation, helping shed light on the nature of the working catalyst. The results show that the oxidation of metallic Pd to PdO is kinetically-controlled at high temperatures, allowing Pd to co-exist along with PdO. We refer to these as metastable Pd ↔ PdO structures. TEM shows that Pd and PdO domains can co-exist within a single particle, forming a phase boundary but allowing both Pd and PdO to be exposed to the gas phase. This kinetically controlled oxidation of Pd explains why we do not see core–shell PdO–Pd structures at elevated temperatures.

Thermally stable Ir/Ce0.9La0.1O2 catalyst for high temperature methane dry reforming reaction

In this study, the use of a thermally stable Ir/Ce0.9La0.1O2 catalyst was investigated for the dry reforming of methane. The doping of La2O3 into the CeO2 lattice enhanced the chemical and physical properties of the Ir/Ce0.9La0.1O2 catalyst, such as redox properties, Ir dispersion, oxygen storage capacity, and thermal stability, with respect to the Ir/CeO2 catalyst. Hence, the Ir/Ce0.9La0.1O2 catalyst exhibits higher activity and stabler performance for the dry reforming of methane than the Ir/CeO2 catalyst. This observation can be mainly attributed to the stronger interaction between the metal and support in the Ir/Ce0.9La0.1O2 catalyst stabilizing the catalyst structure and improving the oxygen storage capacity, leading to negligible aggregation of Ir nanoparticles and the Ce0.9La0.1O2 support at high temperatures, as well as the rapid removal of carbon deposits at the boundaries between the Ir metal and the Ce0.9La0.1O2 support.